The present invention relates as indicated, to the preparation of a coated paper substrate which is particularly useful as a base for electrostatic or electrophotographic sensitive papers, and more particularly to the preparation of base paper having a water and solvent barrier coating thereon and useful in making a lithographic printing plate or offset master for use in electrostatic procedures.
In lithographic printing, a plate is prepared characterized by image areas which are compatible with an oil base ink and hence oleophilic or hydrophobic, and nonimage areas which are oleophobic or hydrophilic. This plate when mounted on a printing press is first wet by a roll carrying a fountain solution which is an aqueous base composition whereby the hydrophilic areas become wet and not receptive to ink, and the oleophilic areas are left uncoated. Thereafter, the plate carrying roll is contacted by the inking roll whereby the nonwetted image areas are coated with an oil base printing ink and the nonimage areas, which repel ink, are left uncoated. In offset printing, the plate roll then comes into pressure contact with a blanket roll to which the ink is transferred in conformity with the image pattern on the plate. The blanket roll is then run in contact with a pressure roll, and a paper web or sheet is introduced between the pressure roll and the blanket roll. The ink from the blanket roll is then transferred to the paper to provide a printed image or final print.
A number of methods has been suggested for preparing an image on a plate to be used in lithographic printing. While direct typing on the plate with a special ink has been used, several procedures have been developed using electrostatic printing principles. According to one of these procedures a plate or sheet including a photoconductive layer extending over its image receiving side is first given a uniform charge over the entire surface in the dark. Then, an image-defining electrostatic charge is formed by exposure of charged photoconductive layer to light through a positive image. The charge is removed in those areas receiving light, and remains where shadow has fallen. This is similar to a photographic process and hence the process is referred to as electrophotographic imaging. Another type of electrostatic process resembles electrographic printing. In this case, an image defining electrostatic charge is laid directly down upon an insulating or dielectric film in a plate. For this purpose, a cathode ray tube, a pin matrix, or a pulsing carona discharge may be used. Alternatively, a direct image transfer may be made from one surface to another. With either of such electrostatic imaging procedures, after the image defining charge has been imposed on the sheet, the image may be developed using a finely divided developer material or toner which deposits on the sheet in those areas where the charge appears. The toner is oleophilic, i.e., readily accepts ink from an inking roll. The sheet or plate with the image developed thereon may be employed as the offset master in a lithographic printing process as described above providing the sheet, either after development, or after further processing, has the proper differentiation between the oleophilic or ink receptive areas (image areas) and the hydrophilic areas or nonimage areas. The latter are receptive to an aqueous medium, such as a fountain solution, but not to ink.
Various types of electrostatic papers and the methods for producing them are well known. For example, reference may be had to U.S. Pat. No. 3,607,255 wherein an offset master is produced using electrostatic imaging procedures. According to this procedure a dielectric film which provides the surface on the plate includes finely divided silica dispersed therein. This yields a very fine roughening on the outer surface of the dielectric film. After the development of the image, a hydrophilic desensitizing composition is spread over the nonimaging areas. The desensitizing composition remains distributed over the nonimage areas during the process of making electrographic prints which involves repeated wetting of the nonimage areas with an aqueous fountain solution followed by application of ink to the image areas.
U.S. Pat. No. 3,787,235 discloses a method of making an electrophotographic sensitive paper.
U.S. Pat. No. 3,653,894 discloses a method of making an electroconductive paper for electrographic recording.
The present invention is concerned with a paper base for such various electrostatic papers and a method of making it.
As indicated above, in actual use, the fountain solution and ink are applied to the master once for each imprint on the paper sheet or web. With masters run for a long time, e.g., more than 1000 copies, it is necessary to protect the paper base (which preferably is a wet strength paper) against the action of the aqueous fountain solution. This protection is usually achieved by providing the paper base with one or more water resistant layers which can be referred to as barrier coatings located between the base and the photoconductive layer. Absorption of the fountain solution into the paper base has several undesirable effects. Primarily, it causes dimensional instability of the fiber in the paper in turn causing distortion of the paper and the image. This results in imperfect registration and wrinkling of the plate. Attack by water on the zinc oxide or photoconductive layer bond as well as on the various subcoatings reduces the strength of these bonds. Eventually, picking off of portions of the zinc oxide coating and/or subcoatings from the master onto the printing blanket causes the loss of image continunity, and/or background fill-in.
Conventional barrier coatings used in the construction of lithographic paper masters are described in U.S. Pat. Nos. 3,298,831 issued to Lau; 3,653,894 issued to Levy; 3,839,033 issued to Matsuno; and 3,787,235 issued to Honjoe. The use of natural and synthetic adhesives to provide surface strength and water resistance to lithographic masters is summarized in TAPPI Monographs 36 and 37, by the Technical Association of the Pulp and Paper Industry (TAPPI) 1 Dunwoody Park; Atlanta, Ga. 30341.
In general, coated printing papers are of two types: Those which are characterized by a high level of pigmentation or filler content in which by virtue of the particulate solids content are not subject to blocking, and low pigmentation or filler content which with low glass transition coatings are subject to blocking.
In the process of preparing a paper base for electrostatic masters, a paper substrate, preferably a precoated paper web having high wet strength, is coated with a resinous film forming composition from an aqueous medium for rendering the paper water resistant. An aqueous dispersion medium is desired not only from the standpoint of ultimate paper properties but also for environmental reasons. Usually the coating is applied to one side by a knifing or blade technique or with the aid of Mayer rods to provide a thin coating. The coating is then dried by passing the impregnated paper through a drier to remove the solvent or dispersing medium. Upon leaving the drier, the web is wound upon a reel. The apparatus is conventional.
It has been found that for best results in fabricating electrostatic papers of various kinds, the base paper should have a predetermined moisture content, generally less than about 8% and desirably about 5%. Where the applied barrier coating is formed from a film forming resin having a high glass transition temperature, dispersion in an aqueous medium is more difficult and may require other agents such as dispersing agents to be present to provide a stable system. These often have an adverse effect on the paper as a base for electrostatic masters. Higher temperatures are required to enable the formation of a film of sufficient impermeability to qualify as a barrier film. This may adversely affect the moisture content of the paper. Resinous film-forming materials having a lower glass transition temperature are desirable because it is easier to control water content of the paper base material.
To facilitate dispersion in an aqueous medium and stability thereof, it is desirable that the film-forming resin contain labile hydrogen such as provided by --COOH, --OH, and the like. Dispersion by a base is convenient, and for purposes of a coating to be applied to paper and dried, a volatile base, e.g., ammonia, ammonium hydroxide and various low molecular weight mono-, di-, and trialkylamines, is especially useful. For most purposes, ammonium hydroxide is used. Such film-forming compositions are very satisfactory for forming base papers for electrostatic masters and formulations based on such film formers can be used as such on individual sheets. The terms "aqueous dispersion" as used herein and in the appended claims will be understood to include colloidal dispersions of extremely fine particle size as are obtainable with ethylene/acrylic acid copolymers in the presence of NH.sub.4 OH as well as latices of relatively larger particle size such as are obtainable with carboxylated styrene-butadiene copolymers.
However, where the coating to be applied contains a relatively low amount of inorganic pigment or filler, i.e., less than 50% by weight on the dry basis, and where the paper is in the form of a continuous web which must be coated, dried and wound upon a spool, a different problem is encountered in which it is a principal objective of this invention to solve. The web temperature on winding upon a take-up reel or spool is often sufficient to permit successive convolutions of the paper to fuse together or "block" whereby the entire roll will solidify into a useless block. The same problem can be encountered to some degree with sheets that are stacked as they leave a dryer. In the case of a paper web, the paper cannot be removed from the reel for subsequent treatment such as the application of the zinc oxide containing dielectric coating composition. While the problem can be alleviated mechanically with chill rolls and/or increased distance between the drier and the end spool, the present invention provides a much simplier chemical method for avoiding the problem. Blocking is not a problem where the coating contains a high proportion of pigment or filler, e.g., more than 50% by weight, dry basis, clay.
It has now been found that an ammonium zirconyl complex may be included in or contacted with the coating formulation to the extent of from about 1% to about 20% by weight (dry basis) of the film forming polymer content and that its presence will prevent blocking. It has also been found most surprisingly that the ammonium zirconyl complex crosslinks the film forming polymers in such a manner that it occurs instantaneously on evaporation of the aqueous vehicle, so that the coated paper emerging from the drier can be rolled or stacked immediately. A particularly surprising result is that the present treatment also provides better imaging characteristics on the finished masters compared to common crosslinking agents such as melamine-formaldehyde (See U.S. Pat. No. 3,317,631) and provides, as well, improvements in water resistance. Moreover, these coatings can be applied at very low weights/ream; for example, as low as 0.5 lbs./side/3300 sq. ft. Higher rates of application up to about 5 lbs./side/3300 sq. ft. may be used if desired.
While the particular utility for these coated papers is in the field of electrostatic masters, such coated paper has other uses, for example as release paper for pressure sensitive labels, beverage labels, etc.